Chain of Gold

Cordelia Carstairs is a Shadowhunter, a warrior with the blood of the angel Raziel. When her father is accused of a crime, she, her mother, and her brother travel to London, where she hopes to prove his innocence. She soon encounters James and Lucie Herondale, her childhood friends, and is drawn into their exciting, glittering, world. All the while she must hide her love for James, who is sworn to marry someone else.
All this is interrupted by a series of demon attacks unlike anything the Shadowhunters have ever seen. They poison whoever they touch, can walk in daylight, and are all but impossible to kill. When London is quarantined, Cordelia and her friends face a horrible choice that will reveal the price of being a hero.